Processes for the molding and shaping of polymer blends comprising thermoplastic polymers blended with reactive materials are generally known. Some thermoplastic polymers are difficult to mold and shape in view of their high processing temperature. By blending the polymers with uncured or low molecular weight reactive type resins, such as epoxy resins, it becomes possible to shape the polymers at lower temperatures. The shaped blend is then cured to obtain the final article.
One such process is described in U.S. Pat. No. 3,763,088 where a non-crystalline thermoplastic resin is blended with uncured epoxy resin. The blends can be prepared by dry blending and fusion mixing the ingredients in a melt extruder at elevated temperatures. The hot melt may be shaped by injection molding. Typically the hot melt is directly injected into the mold. The relative quantity of epoxy resin is chosen such that a final structure results in a continuous phase of polymer particles in which uncured epoxy resin particles have been dispersed. To obtain such a structure it is essential to use epoxy resins with a molecular weight which is sufficiently high. The examples use epoxy resins with molecular weights of 3600 to 8,000.
U.S. Pat. No. 4,623,558 describes a process wherein a thermoplastic resin is blended with an uncured epoxy resin and a thermal initiator or a photo initiator at a temperature below the curing temperature of the system. The blend after combination with glass mats is heated and cured. For polymers with high melting temperatures or high glass transition temperatures, such as polyphenylene ethers, it is however not possible to get homogeneous blends of the polymer, the epoxy resin and the initiator are at temperatures below the curing temperature where the relative amount of thermoplastic resin is too high. In practice, the process of this patent is only suitable for the processing of blends which after curing form a continuous phase of a cured epoxy resin in which particles of the thermoplastic polymer have been dispersed.
U.S. Pat. No. 5,382,384 and its parent U.S. Pat. No. 5,250,228 describe a process wherein a thermoplastic resin, a thermosetting resin, a conductive filler and a curing agent are mixed together wherein the mixture is shaped below the curing temperature of the mixture and wherein the shaped mixture is heated quickly to the curing temperature and is cured at the curing temperature. The mixing is performed below the curing temperature of the curing agent. As described before this makes it impossible for high melting polymers like polyphenylene ether to be used in relatively large quantities.
European patent application EP-B-0 537 005 describes a process for the manufacture of prepregs wherein a polyphenylene ether polymer is blended with a liquid epoxy material, optionally with a flame retardant and a catalyst, at a temperature of 100-130.degree. C. The blend is subsequently granulated into particulates and the particulates are combined with one or more reinforcing cloth or fiber and the thus obtained combination is cured under pressure at 210.degree. C. to 250.degree. C. The blending of the polyphenylene ether and the epoxy are performed at a temperature below the curing temperature of the epoxy resin. This limits the possibilities of the process considerably.
In Polymer, Vol. 35, no. 20, 1994, page 3450 a process is described where solutions of an epoxy and polyphenylene ether (PPE; a thermoplastic resin) are prepared in a Brabender kneader at 175.degree. C. by kneading for about one hour. As an alternative it has been proposed to prepare the blend by mixing for 5-10 minutes in a co-rotating twin screw extrudeN. Subsequently a curing agent is added to the homogeneous solution in the Brabender mixer for about two minutes, the obtained compound is compression molded and cured by curing cycles of two hours followed by a post-cure treatment of four hours at 200.degree. C.
Mixing of epoxy/polyphenylene ether (PPE) mixtures, in particular those with more than 20% by wt of PPE, at temperatures of 175.degree. C. results in compositions which are too viscous for many commercially useful shaping processes such as injection molding.
It has now been found that even with curing agents or catalysts which are fast at the chosen melt mixing temperatures it is possible to work at high temperatures (in the case of PPE at temperatures of over 220.degree. C.) without causing premature reaction of the epoxy resin. This can be achieved by ensuring a quick mixing and complete discharge of the prepared shot. The mixing time in the second step of process of the invention may, depending upon the nature of the constituents and the required temperatures, be far below two minutes.
The present invention provides an injection molding process and apparatus for the shaping of polymer thermosetting materials which are curable to a final shape. With the process and apparatus of the present invention it is possible to use relatively fast curing and/or polymerization systems resulting in short cycle times. With the process and apparatus of the invention it becomes possible to shape polymers at temperatures below their normal processing temperatures. This is of importance for the processing of thermoplastic materials which are unstable or sensitive to oxidation at their processing temperature. With the process and apparatus of the invention it is possible to shape the blend and to cure and/or polymerize the epoxy resin in the blend after the shaping step even when relatively large quantities of thermoplastic resin with a high melting point or a high glass transition temperature are used.